Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system may support time division duplex (TDD) and/or frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the base station to extract transmit beamforming gain on the forward link when multiple antennas are available at the base station. In an FDD system, forward and reverse link transmissions are on different frequency regions.
In a FDD transceiver, the strongest interference on a received signal may be caused by self-jamming leakage from a transmit (Tx) signal that is simultaneously transmitted by the transceiver. The Tx signal may leak to the receive path through the finite isolation (e.g., through a duplexer filter, antenna coupling, circuit card electromagnetic interference (EMI), and VLSI chip coupling). Although in a different frequency band, the Tx leakage signal may cause co-channel interference on the intended received signal due to excitations of some non-linear behavior in the Rx. This scenario is referred herein after as self-jamming. Another case of Tx self-jamming arises when non-linearities present in the transmitter chain generate spectral re-growth commonly indicated as harmonics in the receiver band. For example, spectral re-growth may cause second harmonic distortion (H2D), third harmonic distortion (H3D), and distortions generated by higher order harmonics. The co-channel interference may be generated when nonlinearities are excited in the radio frequency (RF) down conversion components: such as low noise amplifier (LNA), mixer, switches, filters, data converters and other like components.
Two of the nonlinearities that are investigated are second-order inter-modulation distortion (IM2) generated in the Rx chain and third harmonic distortion (H3D) generated in the transmit chain. The problem of transmit signal leakage and subsequent potential IM2 and H3 distortion in a frequency band may be addressed in different ways. A predominant type of solution utilizes analog RF approach which detects presence of an adjacent jammer and increases the current drawn in the mixer to improve linearity. Obviously, this approach results in higher power consumption and hence reduces talk-time of a wireless device. In certain scenarios existing analog solutions may not be enough to keep non-linear distortion signals from negatively impacting the receiver performance. For mitigation of harmonics (e.g., H2D, H3D and other harmonics falling in the receiver band) a high rejection analog filter may be used at the output of the power amplifier (PA). However, this approach results in increased insertion loss, which negatively impacts PA efficiency (e.g., excess current is drawn from the battery). Furthermore, this filter is harmless with respect to coupling mechanism inside the board. In both of these example scenarios an improved solution is desirable.